There has been considerable interest recently in high power microwave (HPM) sources for use in nonlethal directed energy weaponry. The ever increasing reliance on the use of microprocessors that have an increasing density of circuits packaged on a chip causes such systems to be highly vulnerable to HPM attack. Many of the HPM sources that are being developed are derivatives of sources that are well known to the vacuum electronics community. Others are unique to the HPM community, and have no analog in traditional microwave sources.
One non-traditional approach to generating HPM pulses is referred to generally as relativistic beam HPM. In this approach, high voltage capacitors, together with fast switching techniques, are used to generate a short, tailored, high voltage pulse. The pulse is then applied to an electron gun, also known as an electron beam diode. The electron beam diode produces a high-perveance electron beam, where space-charge effects dominate the interaction. The relativistic electron beam, once generated, propagates through an rf interaction region, which converts the beam's kinetic energy to HPM.
However, relativistic-beam-based HPM is limited in both maximum power (10 GW), and lifetime, due to electric field limits in the cavity and substantial cavity erosion. For these and other reasons, the relativistic-beam approach to HPM appears unlikely to provide the mega-Joule energy and terra-Watt power required for various defense missions.
A non-traditional method that avoids the limitations of relativistic-beam HPM is called multi-cycle digital HPM, or MCD-HPM. MCD-HPM is a method of digital microwave generation whereby multiple HPM sources are arrayed and triggered by a plurality of photoconductive switches that are controlled by a single coherent laser source. With reference to FIG. 1A, in one approach a Switch Bypass Source (SBS) circuit is composed of two continuous transmission line plates 100 which are connected to a load at their distant end (not shown). Between the transmission line plates 100 are quarter-wave-long plates 102 forming upper and lower thin film transmission lines (TFTL's), which are charged and are alternately connected by photoconductive switches to the upper and lower transmission line plates 100. In similar approaches, the TFTL's are alternately charged and are all connected to the same plate.
The photoconductive switches 104 are closed one-by-one, beginning with the switch furthest from the load, by sequential application of a laser beam, thereby generating a train of energy pulses of alternating polarity that propagates down the transmission line 100. The switches 104 are closed according to a timing 106 that allows previously generated pulses to pass by each switch before it is closed, which is why the term “switch bypass” is used. Each switch 104 then adds an additional energy pulse to the rear of the passing pulse train.
The timing 106 by which the laser beam is applied sequentially to the photoconductive switches 104 causes the alternating energy pulses to approximate a square wave 108 at a desired microwave frequency. With reference to FIG. 1B, the result is a high power microwave pulse with a duration determined by the number of TFTL's included in the transmission line, and with a frequency determined by the timing 106 of the switch activations. FIG. 1B is a presentation of modeling results for a digitally synthesized equal frequency microwave pulse generated using 66 TFTL's.
Typically, the timing 106 of the photoconductive switch activations is determined by the relative lengths of fiber optic connections that convey the light from the laser source to the various switches. Once the fiber optics have been selected and installed, the output frequency is essentially fixed. Any change to a different microwave output frequency would require a time-consuming, difficult, and expensive process of selecting and installing new fiber optics having different lengths.
It would be desirable to quickly and easily change the output frequency of an MCD-HPM, so that the system could be used to affect a variety of targets having different microwave frequency sensitivities. However, in the prior art there has been no known method for doing so.
What is needed, therefore, is an apparatus and method for quickly and conveniently changing the output frequency of a multi-cycle digital high-power microwave source.